FIGS. 18 to 20 illustrate a boiling water reactor in the related art. In FIG. 18, a nuclear reactor core 2 that is positioned at the center of a reactor pressure vessel 1 is configured of a plurality of fuel assemblies 3. FIG. 19 illustrates a support method of the fuel assembly 3. The fuel assembly 3 is configured to include a fuel rod 4, a spacer 5, an upper tie plate 6, a lower tie plate 7, a channel box 8, or the like. A lower portion of the lower tie plate 7 as a part of the fuel assembly 3 is inserted into four sockets 10 provided in a fuel support 9, and thereby the lower portion of the fuel assembly 3 is fixed. The fuel support 9 is fixed to a core plate 13. An opening 12 is provided at the center of the fuel support 9 such that a cross-shape type control rod 11 can vertically move. An upper portion of the fuel assembly 3 is supported in a horizontal direction by an upper grid plate 14 that is mounted above the nuclear reactor core 2. In FIG. 20, the cross-shape type control rod 11 is disposed at the center of the four fuel assemblies 3, and an installation of neutron instrumentation tube 15 is mounted on the periphery of a location in which the cross-shape type control rod 11 is disposed.
The fuel assemblies 3 are mounted in the reactor pressure vessel 1, in a state in which the core plate 13 and the fuel support 9 below the fuel assemblies 3 are mounted in advance, and the upper grid plate 14 is also mounted above the fuel assemblies 3. Since the bottom of the fuel assembly 3 is supported by the fuel support 9, and the top of the fuel assembly 3 is supported by the upper grid plate 14, it is possible to mount the fuel assembly 3 in the reactor pressure vessel 1 in a self-support manner. Note that the same is true of when the fuel assembly is replaced.
FIGS. 21 and 22 illustrate a fuel assembly of a boiling water reactor including densely arranged fuel assemblies.
As illustrated in FIG. 21, a Y-shape type control rod 16 is disposed between the fuel assemblies 3 that are configured to include a plurality of fuel rods 4 and a hexagonal channel box 8, and the installation of neutron instrumentation tube 15 is disposed at a wing of the Y-shape type control rod 16. FIG. 22 illustrates a support method of the fuel assembly 3. The fuel assembly 3 is configured to include a fuel rod (not illustrated), a spacer (not illustrated), the upper tie plate 6, the lower tie plate 7, the channel box 8, or the like. The lower portion of the lower tie plate 7 as a part of the fuel assembly 3 is inserted into three sockets 10 provided in the fuel support 9, and thereby the lower portion of the fuel assembly 3 is fixed. The opening 12 is provided at the center of the fuel support 9 such that the Y-shape type control rod 16 can vertically move, and the fuel support 9 is fixed to the core plate 13. The fuel assembly 3 is pressed from above at the upper grid plate 14, and thereby the upper portion of the fuel assembly 3 is supported in the horizontal direction via an upper support mechanism 17 provided between the fuel assembly 3 and the upper grid plate 14. In addition, in the fuel assembly 3 of the boiling water reactor including densely arranged fuel assemblies, an outer diameter of the fuel rod 4 is smaller by several millimeters than an outer diameter of the fuel rod 4 used in the fuel assembly of the boiling water reactor in the related art in FIG. 19, and the length of one fuel rod also decreases to have substantially a half of the length. Further, since a weight per one fuel rod decreases to have substantially a third of the weight, the weight of the entire fuel assembly 3 also decreases, compared to the fuel assembly 3 of the boiling water reactor in the related art illustrated in FIG. 19.
In a boiling water reactor disclosed in JP-A-9-281260, a lower portion of a fuel assembly is supported by a fuel support via a lower tie plate and a mass adding member, and an upper portion of the fuel assembly is supported by an upper grid plate via an upper tie plate. In addition, the mass adding member is provided below the lower tie plate so as to prevent the fuel assembly from floating.
A boiling water reactor disclosed in JP-A-8-220276 prevents a fuel assembly from horizontally moving by a tab and a leaf spring provided between the fuel assembly and an upper grid plate, and secures gap holding of the fuel assembly and insertability of a control rod. In addition, a hold down spring is provided between the fuel assembly and the upper grid plate, and thereby the fuel assembly is prevented from floating. In the published invention, the upper grid plate is a member that is necessary for achieving self-support of the fuel assembly and for preventing the fuel assembly from floating. In addition, in a boiling water reactor disclosed in JP-A-2013-145246, the fuel rod is supported by an upper tie plate and a lower tie plate.
In a boiling water reactor disclosed in Japanese Patent No. 4386207, similar to FIG. 18, a method for supporting upper and lower portions of the fuel assembly by an upper grid plate and a fuel support is disclosed. In addition, even a pressurized water reactor disclosed in JP-A-2000-147179 has a structure in which upper and lower portions of the fuel assembly are supported by upper and lower core plates, respectively.
JP-A-6-308269 discloses a support method of a fuel assembly in a fast breeder reactor. Plates that are referred to as an upper core support structure and a lower core support structure are provided below a reactor pressure vessel, the upper and lower plates are connected to a plurality of connection pipes, and a lower portion of a fuel assembly is inserted into the connection pipe such that the lower portion of the fuel assembly is supported. In this manner, self-support of the fuel assembly is secured.